Positioning elements

Relative position and flexbox

UI Toolkit uses a modified version of Yoga, an implementation of the flexbox layouting engine. Flexbox is a common layouting engine found on the web and is controlled via USS style properties. Some of the main style properties are:

• Flex > Direction (flex-direction in USS): Set the direction in which elements are layed out, or the main-axis. The default is column and that means that, without any other overrides, the first child under a parent with this style property will appear above the second child, the second child above the third child, and so on, in a column.
• Flex > Grow (flex-grow in USS): This property defines how an element should grow in the main-axis. It's a ratio that is shared with all other siblings of the same parent. Usually, this property is most useful when trying make an element stretch to take up the entire size of its parents (minus any siblings). To do this, set the Flex > Grow value to 1. If you have two siblings with flex-grow set to 1, they will each take 50% of the parent's available size along the main-axis.
• Align > Align Items (align-items in USS): This property defines how elements should be aligned in the cross-axis, or the perpendicular axis to the main-axis. For example, if you have two Buttons in a VisualElement that has flex-direction: column and align-items: flex-end set, the two Buttons will be squished against the container's right edge. The options for align-items are named like flex-start and flex-end because they depend on the value of flex-direction.
• Align > Justify Content (justify-content in USS): This property defines how elements should be aligned in the main-axis. For example, if you have two Buttons in a VisualElement that has flex-direction: column and align-items: flex-end set, the two Buttons will be squished against the container's bottom edge. The options for justify-content are named like flex-start and flex-end because they depend on the value of flex-direction.

You can toggle some of these flex-related style properties directly in the Viewport using toggles in the header of the blue selection border of any selected element that has child elements:

It is recommend you have a look at how flexbox works in more detail. Any website or tutorial talking about flexbox for the web will also apply to its use in UI Toolkit and UI Builder. A good such resource is:
https://css-tricks.com/snippets/css/a-guide-to-flexbox/

You can create complex dynamic layouts by using a combination of the above flexbox style properties and a hierarchy of VisualElements. Here's an example of a layout that anchors a Button on the bottom-right edge of the screen:
And here's the UXML for this layout, showing the inline styles set on each container VisualElement:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements">
    <ui:VisualElement name="screen-is-blue" style="flex-grow: 1; justify-content: flex-end; background-color: blue;">
        <ui:VisualElement name="toolbar-is-orange" style="align-items: flex-end; background-color: orange;">
            <ui:Button text="Button" display-tooltip-when-elided="true" />
        </ui:VisualElement>
    </ui:VisualElement>
</ui:UXML>

The containers have been colored to reveal their shape. The idea is that you can use nested VisualElement containers to achieve any dynamic layout without resorting to explicitly setting the position and size of each element by hand. This also keeps the layout dynamic and automatically adjusts to the larger container changing size (ie. the screen changing size).

Absolute position

UI Builder also exposes Position style properties. To properly use the Position style properties you must set the Position > Position type to Absolute. This Absolute mode makes an element invisible to the default flexbox-based layouting engine. It is as if it no longer takes any space. Absolute position elements will appear on top of any siblings that are still using Relative position.

It is generally discouraged using Absolute position mode because it bypasses the built-in layouting engine in UI Toolkit and many of the benefits it offers. It can also lead to hard-to-maintain UIs where every element is an absolute island and changing the overall layout would require updating many individual elements by hand. However, there are legitimate uses of Absolute, as shown in the next section.

While using Absolute position, you can use the Position > Left, Top, Right, Bottom style properties to offset and size the element from the respective edges of its parent. Note that you are not setting absolute coordinates on the screen but instead, you are setting offsets relative to the parent element, while the parent element may still be positioned by flexbox using Relative mode. Also, if you set both a Left offset and a Right offset, the Width style property of the element will be ignored and the computed width will now come from the formula:

element-computed-width = parent-width - left-offset - right-offset

Given the above, Left, Top, Right, Bottom can also be interpreted as "anchors". For example, you can "anchor" a Button on the bottom-right of the screen like so:
And here's the UXML showing the inline styles:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements">
    <ui:Button text="Button" style="position: absolute; right: 0; bottom: 0;" />
</ui:UXML>

It is important to note that there is a difference between having Left set to 0 and having Left unset. The former means "set an offset of 0", while the latter means "don't set any offset and let other style properties define the width or height of the element".

You can also modify these offset style properties directly in the Canvas via additional resize handles on each edge and corner of the element's blue selection border. Given how important it is to differentiate between what is set and what is unset, the Canvas also includes "anchor" toggles as orange squares off each side of the element. Furthermore, the Canvas handles will adjust which exact style properties are set when resizing the element visually, depending on which "anchors" are set. For example, say you are resizing the element in the Canvas via its right-border handle:

1. if both Left and Right properties are set, the handle will update the Right property.
2. if only Left is set but Right is not set, the handle will update the Width property.

Using both Relative and Absolute

One of the few legitimate use cases for Absolute position mode is for overlays. Sometimes, you want to overlay some complex UI on top of other complex UI, such as a popup or a dropdown. The idea is to use Absolute position only for the overlay container itself, while continuing to use Relative mode inside the overlay. Here's an example of a simple centered popup:
And here's the UXML for your reference:

<ui:UXML xmlns:ui="UnityEngine.UIElements" xmlns:uie="UnityEditor.UIElements">
    <ui:VisualElement name="complex-ui-screen">
        <ui:Toggle label="Toggle" />
        <ui:MinMaxSlider picking-mode="Ignore" label="Min/Max Slider" min-value="10" max-value="12" low-limit="-10" high-limit="40" />
        <ui:Label text="Label" />
        <ui:Button text="Button" />
        <ui:Button text="Button" />
    </ui:VisualElement>
    <ui:VisualElement name="overlay" style="position: absolute; left: 0; top: 0; right: 0; bottom: 0; background-color: rgba(0, 0, 0, 0.71); align-items: center; justify-content: center;">
        <ui:VisualElement name="popup" style="background-color: rgba(70, 70, 70, 255);">
            <ui:Label text="Exit?" name="message" />
            <ui:Button text="Ok" name="ok-button" style="width: 108.3333px;" />
        </ui:VisualElement>
    </ui:VisualElement>
</ui:UXML>

The main thing to note above is how Absolute position is used. We set all Position > Left, Top, Right, Bottom to 0, meaning 0 pixels away from the edges of the screen. This makes the #overlay element perfectly overlap the #complex-ui-screen container element. For effect, we also set a semi-transparent background to the #overlay element to make the other UI appear darkened. Also note that within #overlay we go right back to using Relative position to center our #popup container VisualElement.